The general forms of most metal detectors which interrogate soils are either hand-held battery operated units, or conveyor mounted units, or vehicle mounted units.
Examples of hand-held products include detectors used to locate gold, explosive land-mines or ordnance, coins and treasure. Examples of conveyor-mounted units include fine gold detectors in ore mining operations. An example of a vehicle-mounted unit is a unit to search for land-mines. These units usually consists of a transmit coil which may transmit an alternating magnetic field associated with a reactive transmit coil voltage, transmit electronics which may generate a transmit signal applied to the transmit coil, and receive electronics which may receive a magnetic field and process received signals to produce an indicator output. The received magnetic field may be detected by a receive coil in most metal detectors, or the transmit coil in some metal detectors such as pulse induction units. The most numerous products of the above examples are the hand-held battery operated products. It is desirable that these have good buried target detection range, especially in magnetic soils which may contain ferrimagnetic materials. Such metal detectors comprise receive electronics which processors a received magnetic field such that the indicator output is responsive to metal targets buried in such soils but not responsive the magnetic soils.
For state-of-the-art metal detectors metal target signals at the limit of the electronic noise produce voltage signals in a receive coil of the order of ten nano Volts. In a metal detector with a “nulled” transmit and receive coil, the varying reactive voltage component from the highly magnetic soils, found in most Australian goldfields for example, may be typically of the order of tens to hundreds of milliVolts across the receive coil. Reactive components are usually symbolised as “X.” Resistive soil components, usually symbolised by “R,” are typically of the order of a hundred times less than the X component. The R soil component is predictable and thus the R soil component may be nulled as described in the patents in the table. As X is poorly correlated with R in magnetic soils and thus unpredictable, the cancellation of the effects of X is essential in order to accurately null magnetic soil signals. X contamination in a resistive demodulated signal is a problem for any metal detector system which transmits non-zero reactive voltages during resistive synchronous demodulation, such as for example multi-frequency sine-waves. One problem of X contamination of resistive synchronous demodulation is non-linear behaviour of receive electronics. In order that this component is less than the smallest detectable metal target signals, the receive electronics must be accurately linear to an order of 106 to 107. This is difficult and especially difficult at tens of kHz or higher. In order that any absolute “phase” inaccuracy of a metal detector system which transmit non-zero reactive voltages during resistive synchronous demodulation does not significantly impede the cancellation of signals from magnetic soils, the absolute X contamination of the resistive components should be about two orders of magnitude less than the absolute resistive components. Hence, the absolute accuracy of the “phase” needs to be of the order of 104. This too is difficult.
The transmit coil may be approximated as an effective inductive component L, in series with an effective resistive component R, which may for mathematical convenience include resistance of cabling and connectors and some elements of the transmit electronics. Thus the transmit coil plus electronics effective resistance has the effective transmit coil time constant τ=L/R. As v=Ld(i)/dt where v is the transmit coil reactive voltage, if the transmit coil current i is constant, then v=0. Thus maintaining the transmit coil current constant for a period results in a zero transmit coil reactive voltage. The applied voltage across the transmit coil is u=v+iR. Hence when the transmit coil reactive voltage is zero, the signal applied across the coil including the transmit coil cable resistance and some resistive elements of the transmit electronics is iR. These equations assume transmit coil, cable and electronics with no stray capacitance or parallel resistance. For a given transmit alternating waveform repeating sequence, and for a constant coupling between the transmit coil and the receive coil, the receive coil voltage waveform will be of constant magnitude and form if the transmit coil alternating reactive voltage waveform v(t) is of constant magnitude and form. As i is proportional to 1/L for a constant transmit coil reactive waveform v(t), the transmit coil current waveform i(t) is modulated by 1/L and so too therefore is Ri(t), and τ too. Thus the applied voltage waveform needs to be modified as the transmit coil inductance is modulated. A method of maintaining a constant reactive transmit coil voltage waveform is described in WO 2005/047932 A1.
Bi-polar transmitting CW systems such as sine-waves and rectangular-waves and examples given below of this invention have an advantage over similar fundamental frequency and power consumption pulse induction systems in that they have intrinsically more gain for signal from targets with long time constants, such as large gold nuggets or unexploded ordnance. For example the resistive component of many CW systems asymptotically decreases as 1/(target time constant) assuming the target has an effective principal time constant, whereas a pulse induction system normally have a response proportional to 1/(target time constant)2.